[Security follow-up C-5] Rate-limit MCP pairing attempts (token bucket, client-side)

[hoainho]

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Tracks follow-up #5 from PR #17 security review. Blocked on mcp-server-v1 M3 (~Jul 28, 2026).

What

MCPPairingPanel currently allows unlimited pairing attempts. An attacker with access to the panel can brute-force tokens at hundreds of requests/sec. Add client-side rate limiting (token bucket) so failed attempts back off.

Why

Even with timing-safe comparison (C-2) and expiration (C-4), an unlimited attempt rate makes brute-force feasible at scale. The server should also rate-limit (defense in depth), but the client-side rate-limit short-circuits attacks before they reach the network — saving server resources AND signaling "stop trying" UX.

Acceptance criteria

• Failed validate token attempts increment a counter stored in chrome.storage.session keyed by pairing_attempts_v1
• After N=5 failures in W=60s, the Submit button is disabled with a countdown timer ("Too many attempts — try again in Xs")
• Successful pairing resets the counter
• Token bucket refills 1 attempt per 12s (so users who fat-fingered the token aren't locked out forever)
• Unit test: 5 failures in <60s → 6th attempt blocked. Wait 12s → 1 attempt restored. Successful attempt → counter reset.

Implementation hint

const MAX_ATTEMPTS = 5;
const WINDOW_MS = 60_000;
const REFILL_INTERVAL_MS = 12_000;

type RateLimitState = {
  failures: number;
  windowStart: number; // ms epoch
};

function checkRateLimit(state: RateLimitState): { allowed: boolean; retryAfterMs?: number } {
  const now = Date.now();
  const elapsed = now - state.windowStart;
  // Refill: 1 attempt per 12s elapsed
  const refilled = Math.min(MAX_ATTEMPTS, state.failures - Math.floor(elapsed / REFILL_INTERVAL_MS));
  if (refilled < MAX_ATTEMPTS) {
    return { allowed: true };
  }
  return { allowed: false, retryAfterMs: REFILL_INTERVAL_MS - (elapsed % REFILL_INTERVAL_MS) };
}

UX hint

When the button is disabled, show the countdown via a useState + setInterval:

{rateLimited && (
  <p className="error">
    Too many attempts. Try again in <span className="countdown">{secondsRemaining}s</span>.
  </p>
)}

Context

• Related PR: feat: improve MCPPairingPanel error messages for invalid token/port (#7) #17 (MCPPairingPanel)
• Blocked by: mcp-server-v1 M3 milestone (server-side rate-limit complements this)
• Severity: Medium-High (security category C-5 in PR feat: improve MCPPairingPanel error messages for invalid token/port (#7) #17 review)
• References: OWASP — Brute Force Attacks